Optical glass fiber must be coated to protect its surface against abrasion, and normal heat-cured coatings are slow curing, so it has been desired to employ ultraviolet-curing coating compositions. This proved to be quite difficult in practice because the optical fiber must be expected to encounter a wide range of service temperatures, including very low service temperatures, and the usual ultraviolet-cured coatings are too hard to begin with or become too hard at the lower temperatures. This excessive hardness causes the difference between the thermal coefficient of expansion of the coating and the thermal coefficient of expansion of the glass to produce microbends in the fiber which interfere with the capacity of the fiber to convey optical messages.
Industry experienced great difficulty in providing ultraviolet curing coatings which would protect the glass surface against mechanical stress without inducing microbending difficulties at low service temperature until our coworker, R. E. Ansel in application Ser. No. 170,148 filed July 18, 1980 found that certain urethane oligomer diacrylates could be combined with appropriate mixtures of monoethylenically unsaturated monomers including a large proportion of a monomer of low glass transition temperature to provide a primer or buffer coating which could then be overcoated with a stronger and harder topcoat to provide the combination of properties which was needed. However, a small proportion of a hydrogen bonding monomer was needed to obtain adequate strength at room and elevated temperature, and this led to high modulus at very low temperature.
Another problem is the desire to employ coatings of high refractive index above 1.48. The optical fiber industry prefers to employ coatings of high refractive index because there is reason to believe the higher index of refraction will cause less attenuation in the light passing through the fiber. The coatings provided in the aforesaid Ansel application provide ultraviolet-cured coatings having the desired high refractive index, and this is an advantage of those coatings.
Unfortunately, the coatings disclosed in the aforesaid Ansel application are only able to resist temperatures down to around -40.degree. C., and in some instances it is desired to extend the service temperature down to around -60.degree. C. While some ultraviolet curing coatings have been found to possess the desired low tensile modulus at low temperature required to resist microbending at -60.degree. C., some of these possess a refractive index below 1.48, and are less desirable for this reason.
One cannot merely select very soft coatings because most of these have very poor properties at room or higher temperature, or have poor heat stability. Optical fibers will not only encounter low service temperatures from time to time, but they will also encounter elevated service temperatures. The buffer coatings must retain some minimal strength at these elevated temperatures, and they must be stable at elevated temperature so as not to acquire greater hardness which induces microbending when low temperatures are subsequently encountered.
This invention intends to provide ultraviolet curing coating compositions which cure to provide buffer coatings for optical glass fibers which exhibit a combination of low tensile modulus which is maintained low at low temperature, high refractive index above 1.48, and reasonable resistance to elevated temperature combined with good stability at such elevated temperature.